1. Field of the Invention
The present invention relates to a Raman amplifier capable of being used for amplification of optical signal in various optical communication systems, and optical repeater and Raman amplification method using such a Raman amplifier, and more particularly, it relates to Raman amplifier, optical repeater and Raman amplification method suitable for amplification of wavelength division multiplexing signals.
2. Related Background Art
Almost all of optical amplifiers used with present optical fiber communication systems are rare earth doped fiber amplifiers. Particularly, erbium doped optical fiber amplifier (referred to as xe2x80x9cEDFAxe2x80x9d hereinafter) using Er (erbium) doped fibers have been used frequently. However, a practical gain wavelength band of the EDFA has a range from about 1530 nm to 1610 nm (refer to xe2x80x9cElectron. Lett,xe2x80x9d vol.33, no.23, pp.1967-1968). Further, the EDFA includes gain having wavelength dependency, and, thus, when it is used with wavelength division multiplexing signals, difference in gain is generated in dependence upon a wavelength of optical signal. FIG. 23 shows an example of gain wavelength dependency of the EDFA. Particularly, in wavelength bands smaller than 1540 nm and greater than 1560 nm, change in gain regarding the wavelength is great. Accordingly, in order to obtain given gain (in almost cases, gain deviation is within 1 dB) in the entire band including such wavelength, a gain flattening filter is used.
The gain flattening filter is a filter designed so that loss is increased in a wavelength having great gain, and the loss profile has a shape substantially the same as that of the gain profile. However, As shown in FIG. 24, in the EDFA, when magnitude of average gain is changed, since the gain profile is also changed as shown by curves a, b and c, in this case, the optimum loss profile of the gain flattening filter is also changed. Accordingly, when the flattening is realized by a gain flattening filter having fixed loss profile, if the gain of the EDFA is changed, the flatness will be worsened.
On the other hand, among optical amplifiers, there is an amplifier referred to as a Raman amplifier utilizing Raman scattering of an optical fiber (refer to xe2x80x9cNonlinear Fiber Optics, Academic Press). The Raman amplifier has peak gain in frequency smaller than frequency of pumping light by about 13 THz. In the following description, it is assumed that pumping light having 1400 nm band is used, and the frequency smaller by about 13 THz will be expressed as wavelength longer by about 100 nm. FIG. 25 shows wavelength dependency of gain when pumping light having central wavelength of 1450 nm is used. In this case, peak of gain is 1550 nm and a band width within gain deviation of 1 dB is about 20 nm. Since the Raman amplifier can amplify any wavelength so long as an pumping light source can be prepared, application of the Raman amplifier to a wavelength band which could not be amplified by the EDFA has mainly be investigated. On the other hand, the Raman amplifier has not been used in the gain band of the EDFA, since the Raman amplifier requires greater pump power in order to obtain the same gain as that of the EDFA. When the pumping light having great power is input to a fiber to increase the gain, stimulated Brillouin scattering may be generated. Increase amplification noise caused by the stimulated Brillouin scattering is one of the problems which makes it difficult to use the Raman amplifier the Japanese Patent Laid-open No. 2-12986 (1990) discloses an example of a technique for suppressing the stimulated Brillouin scattering in the Raman amplifier.
Further, the Raman amplifier has polarization dependency of gain and amplifies only a component (among polarized wave components) coincided with the polarized wave of the pumping light. Accordingly, it is required for reducing unstability of gain due to polarization dependency, and, to this end, it is considered that a polarization maintaining fiber is used as a fiber for amplifying or an pumping light source having random polarization condition.
Furthermore, enlargement of the gain band is required in the Raman amplifier. To this end, Japanese Patent Publication No. 7-99787 (1995) teaches in FIG. 4 that the pumping light is multiplexed with appropriate wavelength interval. However, this patent does not disclose concrete values of the wavelength interval. According to a document (K. Rottwitt, OFC98, PD-6), a Raman amplifier using a plurality of pumping lights having different wavelengths was reported; however, attempt in the viewpoint of the fact that the gain deviation is reduced below 1 dB was not considered.
On the other hand, there is an optical repeater for simultaneously compensating for transmission loss and chromatic dispersion in an optical fiber transmission line, which optical repeater is constituted by combination of an Er doped fiber amplifier (EDFA) and a dispersion compensating fiber (DCF). FIG. 46 shows a conventional example in which a dispersion compensating fiber A is located between two Er doped fiber amplifiers B and C. The first Er doped fiber amplifier B serves to amplify optical signal having low level to a relatively high level and has excellent noise property. The second Er doped fiber amplifier C serves to amplify the optical signal attenuated in the dispersion compensating fiber A to the high level again and has a high output level.
By the way, on designing the optical repeater, it is required that a repeater input level, a repeater output level and a dispersion compensating amount (loss in the dispersion compensating fiber A) be set properly, and, there is limitation that the input level of the dispersion compensating fiber A has an upper limit, because, when the input power to the dispersion compensating fiber A is increased, influence of non-linear effect in the dispersion compensating fiber A is also increased, thereby deteriorating the transmission wave form considerably. The upper limit value of the input power to the dispersion compensating fiber A is determined by self phase modulation (SPM) in one wave transmission and by cross phase modulation (XPM) in WDM transmission. Thus, regarding the optical repeater, an optical repeater having excellent gain flatness and noise property must be designed in consideration of the several variable factors.
FIG. 47 shows a signal level diagram in the repeater. Gain G1 [dB] of the first Er doped fiber amplifier B is set to a difference between an input level Pin [dB] of the repeater and an input upper limit value Pd [dB] to the dispersion compensating fiber A. Gain G2 [dB] of the second Er doped fiber amplifier C is set to (Gr+Ldxe2x88x92G1) [dB] from loss Ld [dB] in the dispersion compensating fiber A, gain Gr [dB] of the repeater and the gain G1 [dB] of the first Er doped fiber amplifier B. Since these design parameters are varied for each system, the values G1 [dB] and G2 [dB] are varied for each system, and, accordingly, the Er doped fiber amplifiers B, C must be re-designed for each system. The noise property in such a system is deeply associated with the loss Ld [dB] in the dispersion compensating fiber A, and it is known that the greater the loss the more the noise property is worsened. Further, at present, a gap from the designed value in loss in the transmission line and the loss in the dispersion compensating fiber A are offset by changing the gains of Er doped fiber amplifiers B, C. In this method, the gain of Er doped fiber amplifier B and C are off the designed value, thus the gain flatness is worsened. A variable attenuator may be used to offset the gap from the designed value of loss. In this method, although the gain flatness is not changed, an additional insertion loss worsens the noise property.
In the optical fiber communication system, although the Er doped optical fiber amplifiers have widely been used, the Er doped optical fiber amplifier also arises several problems. Further, the Raman amplifier also has problems that, since output of ordinary semiconductor laser is about 100 to 200 mW, gain obtained is relatively small, and that the gain is sensitive to change in power or wavelength of the pumping light. So that, when a semiconductor laser of Fabry-Perot type having relatively high output is used, noise due to gain fluctuation caused by its mode hopping becomes noticeable, and that, when the magnitude of the gain is adjusted, although drive current of the pumping laser must be changed, if the drive current is changed, since the fluctuation in the central wavelength is about 15 nm at the maximum, the wavelength dependency of gain will be greatly changed. Further, such shifting of the central wavelength is not preferable because such shifting causes change in joining loss of a WDM coupler for multiplexing the pumping light. In addition, the optical repeater also has a problem that the Er doped optical fiber amplifiers B, C must be re-designed for each system. Further, the deterioration of the noise property due to insertion of the dispersion compensating fiber is hard to be eliminated in the present systems.
In a Raman amplification method for amplifying optical signal by using a stimulated Raman scattering phenomenon, a communication optical fiber is used as an optical fiber acting as an amplifying medium, and, in a distributed amplifying system, a wavelength of pumping light and a wavelength of the optical signal are arranged in 1400 nm-1600 nm band having low loss and low wavelength dependency within a wide band of the communication optical fiber. In this case, regarding the loss of wavelength dependency of the optical fiber as the amplifying medium, a difference between maximum and minimum values is below about 0.2 dB/km in the above band, even in consideration of loss caused by hydroxyl ion (OH) having peak at 1380 nm. Further, even if each pumping powers in a multi-wavelength pumping system are not differentiated according to the wavelength dependence of the loss, the gain of the signals amplified by the pumping lights are substantially the same, there is no problem in practical use.
On the other hand, in a Raman amplifier operating as a discrete amplifier such as EDFA (rare earth doped fiber amplifier) it is necessary to pay attention to the package of the amplifier fiber, for a length of the fiber is about 10 km to about several tens of kilometers in order to obtain the required gain. Thus, it is convenient that the length of the fiber is minimized as less as possible. Although the length of the fiber can be shortened by using an optical fiber having great non-linearity, in the optical fiber having great non-linearity, it is difficult to reduce the transmission loss caused by (OH) generally having a band of 1380 nm, and Rayleigh scattering coefficient becomes great considerably in comparison with the communication fiber, with the result that the difference between the maximum and minimum values of fiber loss within the above-mentioned wavelength range becomes very great such as 1.5 to 10 dB/km. This means that, when the optical fiber having length of 3 km is used, the loss difference due to the wavelength of the pumping light becomes 4.5 dB to 30 dB. Thus, the wavelength division multiplexing signals cannot be uniformly amplified by using the pumping lights having the same intensities.
As one of means for multiplexing the number of pumping lights, there is a wavelength combiner of Mach-Zehnder interferometer type. Since the Mach-Zehnder interferometer has periodical response property regarding frequency, the wavelength of the pumping light must be selected among wavelengths having equal intervals in frequency. Accordingly, the wavelength combiner of Mach-Zehnder interferometer type has limitation in degrees of freedom of wavelength setting, but has an advantage that, when a device of waveguide type or fiber fusion type, if the number of wavelength division multiplexing is increased, insertion loss is not changed substantially.
An object of the present invention is to provide a Raman amplification method capable of uniformly amplifying wavelength division multiplexing signals and suitable to be incorporated as a unit.
Another object of the present invention is to provide a Raman amplifier which can obtain required gain and can reduce wavelength dependency of gain to the extent that usage of a gain flattening filter is not required and which can be used in a band of EDFA.
A further object of the present invention is to apply the Raman amplifier to an optical repeater constituted by an Er doped fiber amplifier (EDFA) and a dispersion compensating fiber (DCF) thereby to provide an optical repeater in which the EDFA is not required to be re-designed for each system and which can compensate dispersion in transmission line loss and/or DCF loss without deteriorating property of the optical repeater.
Further, by Raman-amplifying the DCF, the deterioration of noise property due to insertion of the DCF which could not avoided in the conventional techniques is reduced.
An example of a Raman amplifier according to the present invention is shown in FIGS. 1, 2 and 3. When a small-sized semiconductor laser 3 of Fabry-Perot type having relatively high output is used in a pumping means 1, relatively high gain can be obtained, and, since the semiconductor laser 3 of Fabry-Perot type has a wide line width of an oscillating wavelength, occurrence of stimulated Brillouin scattering due to the pumping light can be eliminated substantially. On the other hand, when a semiconductor laser of DFB type or DBR type, or a Mater Oscillator Power Amplifier (MOPA) is used in the pumping means 1, since a fluctuation range of the oscillating wavelength is relatively small, a gain configuration is not changed by a driving condition. Further, occurrence of stimulated Brillouin scattering can be suppressed by effecting modulation.
Further, by selecting the interval between the central wavelengths of the pumping light to a value greater than 6 nm and smaller than 35 nm, the wavelength dependency of gain can be reduced to the extent that the gain flattening filter is not required. The central wavelength xcexc in this case is a value defined regarding the single pumping light and is represented by the following equation when it is assumed that a wavelength of an i-th longitudinal mode of laser oscillating is xcexi and light power included in that mode is Pi: 
The reasons why the interval between the central wavelengths of the pumping light is selected to a value greater than 6 nm is that the oscillating band width of the semiconductor laser 3 of Fabry-Perot type connected to an external resonator 5 having narrow reflection band width is about 3 nm as shown in FIG. 12, and that a WDM coupler 11 (FIGS. 1, 2 and 3) for combining the pumping lights is permitted to have certain play or margin in wavelength interval between the pumping lights in order to improve wave combining efficiency. The WDM coupler 11 is designed so that lights having different wavelengths are received by different ports and the incident lights are joined at a single output port substantially without occurring of loss of lights. However, regarding light having intermediate wavelength between the designed wavelengths, the loss is increased even whichever port is used. For example, in a certain WDM coupler 11, a width of a wavelength band which increases the loss is 3 nm. Accordingly, in order that the band of the semiconductor laser 3 is not included within said band, as shown in FIG. 12, a value 6 nm obtained by adding 3 nm to the band width of the semiconductor laser 3 is proper for low limit of the interval between the central wavelengths of the pumping light. On the other hand, as shown in FIG. 13A, if the interval between the central wavelengths of the semiconductor laser 3 is greater than 35 nm, as shown in FIG. 13B, a gain valley is created at an intermediate portion of the Raman gain band obtained by the pumping lights having adjacent wavelengths, thereby worsening the gain flatness. The reason is that, regarding the Raman gain obtained by the single pumping light, at a position spaced apart from the gain peak wavelength by 15 nm to 20 nm, the gain is reduced to half. Accordingly, by selecting the interval of the central wavelengths of the pumping light to the value greater than 6 nm and smaller than 35 nm, the wavelength dependency of gain can be reduced to the extent that the gain flattening filter is not required.
According to a second aspect of the present invention, since a Raman amplifier has a control means 4 for monitoring input light or output light with respect to the Raman amplifier and for controlling pump powers of the pumping means 1 on the basis of a monitored result to maintain output light power of the Raman amplifier to a predetermined value, given output can be obtained regardless of fluctuation of input signal power to the Raman amplifier and/or dispersion in loss of a Raman amplifier fiber.
According to a third aspect of the present invention, since a Raman amplifier has a controlling means 4 for flattening Raman gain, the gain can be flattened. Particularly, as shown in FIG. 16, by monitoring lights having wavelengths obtained by adding about 100 nm to the wavelengths of the pumping lights, respectively, and by controlling the powers of the pumping lights to order or align the monitored light powers, the gain can be flattened. Further, since a wavelength stabilizing fiber grating (external resonator 5) which will be described later can suppress the shift in the central wavelength due to change in drive current of a Fabry-Perot type semiconductor laser, it can also be used as a means for permitting control of the gain.
According to a fourth aspect of the present invention, since a Raman amplifier has a control means 49 for monitoring input signal power and output signal power and for controlling pumping light power to make a ratio between the input signal power and the output signal power constant thereby to maintain gain of the Raman amplifier to a predetermined value, given output can be obtained regardless of fluctuation of the input signal power to the Raman amplifier and/or dispersion in loss of a Raman amplifier fiber.
According to a fifth aspect of the present invention, in a Raman amplifier, since an optical fiber having non-linear index n2 of refraction of 3.5E-20 [m2/W] or more is used as an optical fiber 2, adequate amplifying effect can be obtained, from the result of investigation.
According to a sixth aspect of the present invention, in a Raman amplifier, since the optical fiber 2 exists as a part of a transmission fiber for propagating the optical signal, the amplifier can be incorporated into the transmission optical fiber as it is.
According to a seventh aspect of the present invention, a Raman amplifier utilizes a part of a dispersion managed transmission line and can constitute an amplifier as it is as a amplifying medium.
According to an eighth aspect of the present invention, in a Raman amplifier, since the optical fiber 2 exists as an amplifier fiber provided independently from a transmission fiber for propagating the optical signal and inserted into the transmission fiber, an optical fiber suitable for Raman amplification can easily be used for the optical fiber 2 and the chromatic dispersion compensating fiber can easily be utilized, and a discrete amplifier can be constituted.
In an optical repeater according to the present invention, since loss of an optical fiber transmission line 8 is compensated by using the Raman amplifier, an optical repeater having the function of the Raman amplifier can be provided.
Among optical repeaters of the present invention, in a repeater in which rare earth doped fiber amplifier(s) 10 is (are) provided at preceding or following stage or at both stages of the Raman amplifier, since the loss of the optical fiber transmission line 8 is compensated by the Raman amplifier 9 and the rare earth doped fiber amplifier(s) 10, desired amplifying property suitable for various transmission systems can be obtained.
Among optical repeaters of the present invention, in accordance with an arrangement in which the Raman amplifier 9 and the rare earth doped fiber amplifier 10 are combined, a repeater adapted to various systems can be obtained. This fact will be explained hereinbelow as an example that DCF is used as the amplifier fiber of the Raman amplifier 9. FIG. 17 shows design parameters of a conventional optical repeater, and the gains G1, G2 are varied for each system. Further, it is not inevitable that input of the repeater and loss of the DCF are fluctuated by scattering in repeater spacing and scattering in the DCF. Such fluctuation is directly associated with the fluctuation of the gain of the EDFA, which fluctuation of the gain leads to deterioration of flatness. FIG. 18 schematically shows a relationship between the flatness and the gain of the EDFA. Since the flatness is optimized by limiting the used band and the average gain, if the average gain is deviated from the optimum point, the wavelength dependency of gain is changed to worsen the flatness. In order to avoid the deterioration of the flatness, the gain of the EDFA must be kept constant. Conventionally, a variable attenuator has been used as a means for compensating the fluctuation in input level and loss of the DCF. FIG. 19A shows an example that an attenuating amount of the variable attenuator is adjusted in accordance with the fluctuation of the input level to control the input level to the DCF to be kept constant and FIG. 19B shows an example that an attenuating amount is adjusted in accordance with the fluctuation in loss of the DCF to control intermediate loss to be kept constant. In both examples, the gains of two amplifiers are constant. However, in these methods, since useless loss is added by the variable attenuator, there is an disadvantage in the viewpoint of noise property. In the present invention, by compensating the change in design parameters of the repeater by the Raman amplification of the DCF, the gain of the EDFA is kept constant, requirement that the EDFA be re-designed for each system is eliminated, and the scattering in repeater spacing and scattering in the DCF can be compensated without sacrificing the flatness and the noise property. FIG. 20 shows design values of the EDFA when the Raman amplification of the DCF is applied to the specification of the repeater of FIG. 17. By selecting the Raman gain of the DCF appropriately, the properties of the EDFA required for three specifications can be made common. Further, as shown in FIGS. 21A and 21B, the fluctuation in input level and in loss of the DCF can be compensated by changing the Raman gain without changing the gain of the EDFA. In any cases, the Raman gain is adjusted so that the output level of the DCF becomes constant, while keeping the gain of the EDFA constant. Further, by compensating the loss of the DCF itself by the Raman amplification, the deterioration of the noise property due to insertion of the DCF which could not avoided in the conventional techniques can be reduced. FIG. 37 shows measured values of a deteriorating amount of the noise figure when the DCF is inserted and of a deteriorating amount of the noise figure when the Raman amplifier using the same DCF is inserted.
In the optical repeater according to the present invention, when a Raman amplifier using an pumping light source in which wavelengths are not combined is provided, although an operating wavelength range is narrower, a construction can be simplified and the same property as the aforementioned optical repeaters can be obtained, except for the band width, in comparison with an optical repeater having a Raman amplifier pumped by a plurality of wavelengths. FIGS. 38 and 39 show measured examples of the optical repeater using the Raman amplifier pumped by the pumping light source in which wavelengths are not combined and of the optical repeater using the Raman amplifier pumped by the plurality of wavelengths.
In the Raman amplifier according to the present invention, when a difference between maximum and minimum values of the central wavelength of the pumping light is within 100 nm, overlapping between the pumping light and the optical signal can be prevented to prevent wave form distortion of the optical signal. If the wavelength of the pumping light is similar to the wavelength of the optical signal, since the wave form of the optical signal may be deteriorated, the wavelength of the pumping light and the wavelength of the optical signal must be selected so that they are not overlapped. However, in a case where the pumping light has band of 1.4 xcexcm, when the difference between the maximum and minimum values of the central wavelength of the pumping light is smaller than 100 nm, as shown in FIG. 14, since the difference between the central wavelength of the gain caused by one pumping light and the wavelength of such pumping light is about 100 nm, the overlapping between the wavelength of the pumping light and the wavelength of the optical signal can be prevented.
In the Raman amplifier according to the present invention, when the pumping lights having adjacent wavelengths are propagated through the optical fiber 2 toward two different directions so that the optical signal pumped bi-directionally, the wavelength property required in the WDM coupler 11 shown in FIG. 1 and FIGS. 2 and 3 can be softened. The reason is that, as shown in FIG. 15, in all of pumping lights from both directions, although the central wavelengths become xcex1, xcex2, xcex3, xcex4 and the interval is greater than 6 nm and smaller than 35 nm, when considering the pumping lights from only one direction, the central wavelengths become xcex1 and xcex3 or xcex2 and xcex4 and the wavelength interval increased to twice, with the result that the property required in the WDM coupler 11 can have margin or play.
In the Raman amplifier according to the present invention, when the wavelength stabilizing external resonator 5 such as fiber grating is provided at the output side of the semiconductor laser 3 of Fabry-Perot type, noise due to fluctuation of caused by mode hopping of the semiconductor laser 3 of Fabry-Perot type can be suppressed. In consideration of one pumping light source, it makes the bandwidth narrower to connect the wavelength stabilizing external resonator 5 to the semiconductor laser 3. Since it also results the smaller wavelength interval in case of combining pumping light sources by the WDM coupler 11 (FIGS. 1, 2 and 3), pumping light having higher output and wider band can be generated.
In the Raman amplifier according to the present invention, when the pumping light of the semiconductor laser 3 is used to be polarization-combined for each wavelength, not only polarization dependency of gain can be eliminated but also the pump power launched into the optical fiber 2 can be increased. In the Raman amplification, only the components matched with the polarized of the pumping light can be given the gain. When the pumping light is linear-polarized and the amplifier fiber is not a polarization maintaining fiber, the gain is changed due to fluctuation of relative state of polarization of the signal and the pumping light. Therefore, the polarization dependence of gain can be eliminated by combining two linear-polarized pumping lights so that the polarization planes are perpendicular to each other. Further, it increases the pumping light power launched into the fiber.
In the Raman amplifier according to the present invention, in the case where a wavelength combiner of planar lightwave circuit based on a Mach-Zehnder interferometer is used as a means for wave-combining MOPA or a semiconductor laser of Fabry-Perot type, DFB type or DBR type having a plurality of wavelengths, even when the number of the wavelengths to be combined is large, the wave-combination can be achieved with very low loss, and pumping light having high output can be obtained.
In the Raman amplifier according to the present invention, as shown in FIG. 6, when a polarization plane rotating means 7 for rotating the polarization plane by 90 degrees is provided so that the optical fiber 2 simultaneously includes the plurality of pumping lights generated by the pumping means 1 and the pumping lights having a orthogonal state of polarization to the pumping lights generated by the pumping means 1, in principle, given gain can always be obtained regardless of the state of polarization of the optical signal, with the result that the band of the Raman gain can be widened.
In the optical repeater according to the present invention, when the Raman amplification is utilized by coupling residual pumping light of the Raman amplifier to the optical fiber transmission line 8, a part of loss of the optical fiber transmission line 8 can be compensated.
In the optical repeater according to the present invention, when the residual pumping light of the Raman amplifier is utilized as pumping light for the rare earth doped fiber amplifier 10, the number of the semiconductor lasers to be used can be reduced.
In the optical repeater according to the present invention, when a dispersion compensating fiber is used as the optical fiber 2 of the Raman amplifier 9, the chromatic dispersion of the optical fiber transmission line 8 can be compensated by the Raman amplifier 9, and a part or all of the losses in the optical fiber transmission line 8 and the amplifier fiber 2 can be compensated.
According to a twenty-ninth aspect of the present invention, in a Raman amplification method by stimulated Raman scattering in an optical fiber through which two or more pumping lights having different central wavelengths and said optical signals are propagated, the pumping power launched into said optical fiber increases as the central wavelengths of said pumping lights is shorter.
According to a thirtieth aspect of the present invention, in a Raman amplification method by stimulated Raman scattering in an optical fiber through which two or more pumping lights having different central wavelengths and said optical signals are propagated, total pumping power on the shorter wavelength side with respect to the center between the shortest and longest central wavelengths among said two or more pumping lights is greater than on the longer side.
According to a thirty-first aspect of the present invention, in a Raman amplification method by stimulated Raman scattering in an optical fiber through which three or more pumping lights having different central wavelengths and said optical signals are propagated, the number of the pumping light sources on the shorter wavelength side with respect to the center between the shortest and longest central wavelengths among said three or more pumping lights is greater than on the longer side, and the total pumping power on the shorter wavelength side is greater than on the longer side.
To achieve the above object, according to thirty-second to thirty-fourth aspects of the present invention, when the shortest pumping wavelength is defined as a first channel and, from the first channel, at respective intervals of about 1 THz toward the longer wavelength, second to n-th channels are defined, pumping lights having wavelengths corresponding to the first to n-th channels are multiplexed and pumping light having a wavelength spaced apart from the n-th channel by 2 THz or more toward the longer wavelength is combined with the multiplexed light, and light obtained in this way is used as the pumping light of the Raman amplifier. When the shortest pumping wavelength is defined as the first channel and, from the first channel, at respective intervals of about 1 THz toward the longer wavelength, the second to n-th channels are defined, light obtained by combining all of the wavelengths corresponding to the channels other than (nxe2x88x921)-th and (nxe2x88x922)-th channels is used as the pumping light of the Raman amplifier. Alternatively, light obtained by combining all of the wavelengths corresponding to the channels other than (nxe2x88x922)-th and (nxe2x88x923)-th channels is used as the pumping light of the Raman amplifier.